The mapping camera

The SIM bay’s mapping camera was really two cameras in one package, with a third instrument included to aid interpretation of the imagery. It was based on a wide-field camera designed in the 1960s as part of the then-secret Corona reconnaissance satellite programme to provide context images of target sites. The main instrument was the metric camera, a conventional photographic imager with a 76-millimetre lens that took wide-angle, often spectacular, square images on 127-millimetre wide film with a maximum resolution on the lunar surface of about 20 metres. Like the surface Hasselblads, the camera had a Reseau plate to imprint tiny crosses onto the image to allow researchers to compensate for changes in the film’s geometry over time. A large cassette carried over 450 metres of film which was sufficient for at least 2,500 images.

When using such imagery for mapping purposes, it was vital to know the direction in which the camera was pointed. This information was supplied by the associated stellar camera. At the same time as a frame was being exposed by the metric camera, another was taken of the stars looking to the side. Since researchers knew the precise angle between the axes of the two optical systems, they could deduce exactly where in space the metric camera was aimed. To facilitate the sideways view of the stellar camera, the entire mapping camera system was mounted on a track that allowed it to extend out of the service module bay.

The third part of the mapping camera system, the laser altimeter, determined the distance between the camera and the lunar surface to an accuracy of about one metre. This worked by sending extremely brief pulses of laser light to the surface along a line parallel to the metric camera’s axis. A detector then received the pulse reflected by the surface and accurately timed its return in order to obtain the distance. Altitude information was sent to Earth by telemetry and, when carried out simultaneously with a metric camera frame, was photographically coded onto the

film. Both the stellar camera and the laser altimeter could continue operating while over the Moon’s darkened hemisphere – that is, the stellar camera served to locate the terrain sampled by the laser pulses in the darkness.